Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Optical networks use Wavelength Selective Switches (WSS) to dynamically route optical wavelength signals from a source to a destination. WSS devices often rely on wavelength manipulation elements such as liquid crystal on silicon (LCOS) devices or micro-electromechanical (MEMS) mirror arrays to perform the routing.
LCOS devices, such as those utilized in U.S. Pat. No. 7,092,599 (Frisken), include a liquid crystal material sandwiched between a transparent glass layer having a transparent electrode, and a silicon substrate divided into a two-dimensional array of individually addressable pixels. Each pixel is individually drivable by a voltage signal to provide a local phase change to an optical signal, thereby providing a two-dimensional array of phase manipulating regions. Similarly, MEMS based WSS devices, such as those utilized in PCT Application Publication WO 02/075410 (Wilde et al.), include a 2-dimensional array of individually tiltable mirrors. Each mirror operates on the local optical wavefront to selectively steer and manipulate optical signals incident thereon.
Manipulation of individual spectral components is possible once an optical signal has been spatially separated by a diffractive element such as a diffraction grating. The spatial separation of spectral components is directed onto predetermined regions of the LCOS device or MEMS array, which can be independently manipulated by driving the corresponding pixels/mirrors in a predetermined manner.
It is known to use LCOS devices for applications such as optical component emulation, (dynamic) gain shaping, channel add/drop or channel routing, and FLEXGRID™: dynamic channel control and group delay/dispersion adjustment. For example, U.S. Pat. No. 7,457,547 (Frisken et al.) entitled “Optical calibration system and method”, which is incorporated herein by way of cross-reference, discloses a system and method for calibrating an LCOS device to compensate for phase distortions and to reduce cross-talk. LCOS devices can be used to modify or selectively attenuate the channel spectrum by driving selected pixels with a sinusoidal signal or by varying the local angle θ of the phase steering function.
One issue that is encountered in optical systems is the alteration or degradation of the spectral shape of wavelength channels transmitted through the system. In traversing each optical component, such as a WSS, the spectral shape of a wavelength channel is modified, depending upon the filter function of that device. This is particularly prominent in systems implementing twin reconfigurable add/drop multiplexer (ROADM) architectures wherein each ROADM includes a first WSS for dropping channels and a second WSS for adding channels. When compared to traditional broadcast and select architectures, these twin architectures provide improved channel blocking and isolation. However, they suffer from effects arising from an additional WSS being present.
One particular detrimental effect is the narrowing of a channel bandwidth upon transmission through a WSS, as reported in Pulikkaseril et al., “Spectral Modeling of Channel Band Shapes in Wavelength Selective Switches”, Optics Express, Vol. 19, No. 9, 25 Apr. 2011. Cascading WSS devices has the net effect of multiplying the filter spectra together. Therefore, this narrowing effect is amplified when a particular wavelength channel is transmitted through a series of concatenated WSS devices, as is encountered in many optical networks.
In optical component emulation, the bandwidth of an isolated wavelength channel can be controlled for testing purposes by adjusting the number of LCOS pixel columns which are directed towards an outgoing port. However, in WSS devices, the channel plan can be fully loaded and the channels closely packed together across the LCOS pixel matrix. Slight additional pixel space can be provided by sharing pixel columns between two adjacent channels. Further, dividing columns between two channels can aide in centering the channel on the International Telecommunications Union (ITU) grid. This sharing is possible by setting the pixels of that column such that a first sub-column of pixels couples a first channel but attenuates the second, while a second sub-column couples the second channel but attenuates the first. However, even implementing this “partial columns” technique, the channel bandwidth cannot be extended beyond the edge of the adjacent channels.
Therefore, there is a desire to more adequately compensate for this bandwidth narrowing effect in WSS devices.